Current control system



Sept. 5, 19,44. w. s. 'rHoMPsoN' CURRENT CONTROL SYSTEM y Filed March 24, .194:2l

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ATTORNEY Patented sepas, 1944 VcuRRnrrr CONTROL SYSTEM Walter S. Thompson, Los Angeles, Callf.assign or to Radio Corporation of America, a corporation of Delaware Application March 24, 1942, SerialNo. 436,006 .10 Claims. (CL 179-1003) This invention relates to electrical current transmission systems and particularly toa control syste'm for the recording of sound or for unit being timed in amanner to bev described hereinafter.

radio transmitters. In sound recording for motion pictures, the means provided inV accordance with the invention will control the noise reduction action, while in radio broadcasting the in` vention is applicableV to the control of the carrier current amplitude. l n l y Practically al1 present day sound recording systems embody noise reduction to increase the iilm volume range by increasing the lower recordable limit. Several forms of noise reduction are in general use, one typebeing applicable tovariable area. sound tracks wherein theL portion of thel sound track area not utilized by the modulations is made opaque, and another type `being 'The type of compressoremployed may be as shown in Singer Patent No. 2,255,683, of September 9, 194.1. Although this patent-. discloses a compressor, it is only necessary to interchange the output leads from the rectiiler' to obtain an expanding action, as is well known in the art. The present invention may thus employ two such units in series prior to the noise reduction amplier, the units being j connected into a standard sound recording channel either after the recorder amplier or between the regular signal compres sor and the recorder amplier. For radio broadcasting, the invention may be employed to vary the amplitude ofya.l carrier in accordance with the variations in the signal strength to insure applicable to variable density sound tracks wherein the average 'density ofthe track is varied in accordance with the signal level.

In substantially every form of noise reduction, a rectier is employed for obtaining a direct current proportional to the level or volume of the signal. This direct current is then used eitheruto operate noise reduction elements in the form of shutters to vary the` light impressed on the film, yor to vary the average value of the intensity of the light impressed upon the nim by suitable means. f

In kapplying the noise reduction action, itis desirable Ythat the clipping o'f the first peaks of the signal modulations. particularly when the beginning of the signal has a considerable ampliD tude, be prevented, ywhile subsequently securing the kmaximum amount of noise reduction during steady state conditionsi` That is, in the case of variable area sound tracks,l the boundary vor boundaries of` the .opaque portions should substantially coincide or follow the peaksof the modulations, theamount of separation between the opaque portionland the peaks being known as marginf Thus, what is4 desired is a low mar-f gin to'provide adequate noise reduction during steady state conditions, and a fast action at the beginning of the signal to avoid peak clipping.

The present types of noise reduction systems which are fast enough yto avoid peak clipping have -anA undesirably large margin, while the low margin systems are slow acting and will clip the peaks at the beginning of the sound waves. The present invention is directed to a system which will provide a zero or substantially zero margin, while producing the necessary rapidity of action. This is accomplished by a series' arrangement of an expander and'compreslorahead of the noise reduction ampiiflen'each that thek carrier will at all times accommodate l the peaks ofthe broadcast signal.

The principal object of the invention, therefore, is to provide means for controlling a current asa. function of the volumeof a signal.

Another object of the invention is to provide means for utilizing the action of a serial arrangement of an expanderand compressor to control a characteristic or function of a signal current.y

A further object vof the invention is to provide means for controlling the noise reduction action ofl a sound recording channel so .as to accurately` follow the peaks of the signal.

l Another object of the invention is to provide a sound recording channel with afast-acting and low margin noise reduction system.

, sA further object of the'invention is to provide means for operating the noise reduction shutters lof a variable area sound recording system so as to follow thepeaks ofthe recorded modu-f lations. Although they-novel features whichpare believed to be characteristic ofthe invention are pointed out with pamcmarity in the emms ap-l l pended herewith. the manner of its organization and the mode of its operation will be better understoodby referring to the following description read in conjunction with the accompanying y drawing forming a part hereof, in' which:

Figure 1 isl a block diagrammatic drawing showings. standard sound recording channel emn bodying the invention.

Fig. 2 is a partialsection cfa sound track. illustrating the operation of the invention, and.

Fig. 3 is a graph showing the timing operation of the serially arranged expander andcompressor units of the invention.

Referring now to Ii'ig.` 1,fa microphone I feeds .signal compressor is a. control expander I5 connected to a control compressor I6, the output of the latter being fed to a noise reduction ampliiler I1 connected to a noise reduction unit I8 having shutters I9.

The sound recording portion of the system is the well-known variable area type in which a e light from a lamp 20 is collected by a lens 2| and projected upon an aperture mask 22. The

Y light passing through the aperture of the mask 22 and past the shutters I9 is projected by a lens 25 upon a. mirror of the galvanometer I3, from which it is reected to a slit mask 26 through alens 2l. The light emerging through the sl-it of the mask 26 is projected on the lm 30 by projection lenses 3l. Such a sound recording system is disclosed and claimed in U. S. Patents Nos. 2,102,776, 2,102,777 and 2,102,778.

The above-described lm recorder will produce a Vbilateral variable area track well-known in the art, a portion of which is shown in Fig. 2, to which reference will now be made. During the periods of no signal short light beams will be projected to the film 30, as shown by the bias lines :c of the sound track of the type suitable for reproduction in theaters. The outside boundaries of the bias lines are, of course, produced by the edges of the shutters I9, the shutters moving outwardly and inwardly to accommodate the modulations as the signal increases and decreasesin amplitude, respectively. As illustrated by the full shutter lines, it will be noted that the shutter movement is such that the rst two peaks at the beginning of the signal are clipped before the shutters reach point c of their steady state condition. However, from then on, the action is such as to provide a zero 0r substantially zero margin until the amplitude increases, the shutters clipping the first peak of the increased signal until point e is reached. Thereafter, the noise reduction shutters just accommodate the peaks of the signal during the next steady state condition.

To provide a suillciently rapid noise reduction action to avoid clipping the peaks of the beginning of a signal or signal amplitude increases, Vit will be necessary to have the shutters move as shown by the vdotted line, which however, provides too large a margin during steady state conditions. Such a large margin is undesirable because of the introduction4 of noise by the clear portion of the nlm between the dotted line and the solid line of low margin shutter action. n It is realized, therefore, that the preferred action is a combinationof the full and dotted lines as shown by the dot and dash line connection therebetween. By this combination of actions, the shutters will move fast enough at the beginning o1 or during an increase in a signal so as to clear the rst peaks as shown by the dotted line, slow down as shown by the dot and dash lines to provide a substantially zero margin as shown by fulll lines. With the invention, this desired result may be accomplished.

To explain how the shutter action is obtained, reference is made to Fig. 3, wherein is charted the gain of each individual expander and compressor, as Well as the resulting gain of the combination with respect to time. The noise reduction amplier will iirst be adjusted to provide a substantially zero margin for steady state conditions as shown by the full shutter lines in'Fig. 2. The expander-compressor combination will then be adjusted to have a zero gain during these steady state conditions, wherein the amount of compression equals the amount of expansion. The starting rate for each particular compressor and expander will now be adjusted as shown by curves A and B in Fig. 3, curve A being the expander characteristic and curve B the compressor characteristic. Starting at the point a correspondingto point a in Fig. 2, at the beginning of a signal, the gain of the expander is adjusted to rapidly increase with increasing signal level while the gain of compressor is adjusted to slowly decrease with the same increase in signal level.

The result is a rapid increase in overall gain of the combination, as shown by curve C, thus producinga rapid shutter action as shown in Fig. 2, between points a and b. At 'point b in Fig. 3, where the distances g and g are equal the overall curve C begins to drop, which shows a decrease in overall gain to point c Where the distances h and h.' are equal and the compression balances the expansion. The normal low margin adjustment of the noise reduction amplifier is then effective.v This normal noise reduction action will continue' until there is a further increase in signal as'shown by the continuation of curves A, B and C, points d and e in Fig. 3 corresponding to points d and ein Fig. 2, the distance i being equal to distance i' showing equal compressor and expander actions. By causing the output of the noise reduction amplifier to respond in accordance with curve C the shutter or shutters or other noise reduction element will follow the combination curve in Fig. 2, and just clear the peaks of the waves.

The time axis in Fig. 3 does not exactly correspond to the time of film movement between points a and e in Fig. 2 since the length of portion c of curve C and corresponding rportions of curves A and B in Fig. 3 are not related to the length of iilm between the two points labelled c in Fig. 2. As the gain of the combination of compressors I5 and I6 is zero during the time when the signal amplitude is constant, portion c in Fig. 3 was not extended beyond the length shown. However, the portions of Figs. 2 and 3 where the amplitude is increasing may be considered to correspond on asimilar time basis.

In the above described example, it is toA be noted that no closing action is shown. Although the gain of the combination expander-compressor is preferably zero during the closing time and the noise reduction unit I1 has the normal slow rate of closing, as shown in Kreuzer Patents No. 2,224,914, of December 17, 1940, and No. 2,260,717, of October 28, 1941, a variation in closing time may be easily obtained if desired by providing each unit with a diierent closing adjustment.`

It is also to be understood that although the expander and compressor are shown as self-contained units, a single rectiiier could control the variable gain ampliiiers in both units through different timing circuits.

I claim as my invention:

1. In a current control system, the combination of a source of electrical currents, means for obtaining an average current proportional lto the source of signal currents, means for deriving an amplitude of said currents, and means for varying said average current to follow the peak values o! said electrical currents during increases in amplitude of said currents.

2. Al current control system in accordance with claim 1 in which said last-mentioned means comprises an expander-compressor combination having a substantially zero overall gain for said electrical currents of constant amplitude, said compresser having a different operating time from that oi said expander for current variations increasing in amplitude.

3. A current control system in accordance with claim l in which said last-mentioned means comprises an expander-compressor combination having a substantially constant gain for said electrical currents ot constant amplitude, said expander having a faster operating time from that of said compressor during increases in amplitude or said electrical currents.

4. A sound recording system comprising a source of signals to be recorded, means for modulating a light beam in accordance with the instantaneous amplitudes o1' said signals, means for deriving an average current having a certain proportionality with respect to the amplitude of said signals for further modulating said light beam, and means intermediate said source -oi signals and said last-mentioned means for aifecting said proportionality of said average currentwith respect to the amplitude of said signals during amplitude increases therein.

5. A sound recording system in accordance with claim 4 in which said intermediate means comprises an expander-compressor combination having a substantially zero gain during steady state conditions of said signals.

6. A signal transmission system comprising ar average current having a certain proportionality with respect to the volume variations of said currents. and additional means for varying said proportionality oi said average current during amplitude increases in said signal currents.

7. A signal transmission system in accordance with claim 6 in which said last-mentioned means includes an expander having a rapid operating time and a compressor having a slower operating time, the combination eiiect oi said expander and compressor for signal currents of constant amplitude being substantially zero.

8. A noise reduction system for recording sound on film comprising a rectier for signal currents kto be recorded, means for providing said rectifier with a certain predetermined operating time for increases and decreases in amplitude oi said signal currents, and means feeding said rectifier for varying said certain predetermined timing of said rectifier during amplitude increases in said signal currents. f

9. A noise reduction system in accordance with claim 8 in which said rectiiler feeding means includes means for increasing the rapidity of the operating time of said rectifier during amplitude increases in said signal currents while maintaining said operating time as predetermined during steady state conditions.

10. A noise reduction system in accordance with claim 8 in which said rectiiier feeding means includes a compressor and an expander connected in'series, said compressor operating on signal increases at a slower rate than said expander, the combined gain of said compressor and expander being zero when transmitting currents of constant amplitude.

- WALTER S. THQMPSON. 

